SLC13A3 variants cause acute reversible leukoencephalopathy and α-ketoglutarate accumulation.

OBJECTIVE: SLC13A3 encodes the plasma membrane Na+ /dicarboxylate cotransporter 3, which imports inside the cell 4 to 6 carbon dicarboxylates as well as N-acetylaspartate (NAA). SLC13A3 is mainly expressed in kidney, in astrocytes, and in the choroid plexus. We describe two unrelated patients presenting with acute, reversible (and recurrent in one) neurological deterioration during a febrile illness. Both patients exhibited a reversible leukoencephalopathy and a urinary excretion of α-ketoglutarate (αKG) that was markedly increased and persisted over time. In one patient, increased concentrations of cerebrospinal fluid NAA and dicarboxylates (including αKG) were observed. Extensive workup was unsuccessful, and a genetic cause was suspected. METHODS: Whole exome sequencing (WES) was performed. Our teams were connected through GeneMatcher. RESULTS: WES analysis revealed variants in SLC13A3. A homozygous missense mutation (p.Ala254Asp) was found in the first patient. The second patient was heterozygous for another missense mutation (p.Gly548Ser) and an intronic mutation affecting splicing as demonstrated by reverse transcriptase polymerase chain reaction performed in muscle tissue (c.1016 + 3A > G). Mutations and segregation were confirmed by Sanger sequencing. Functional studies performed on HEK293T cells transiently transfected with wild-type and mutant SLC13A3 indicated that the missense mutations caused a marked reduction in the capacity to transport αKG, succinate, and NAA. INTERPRETATION: SLC13A3 deficiency causes acute and reversible leukoencephalopathy with marked accumulation of αKG. Urine organic acids (especially αKG and NAA) and SLC13A3 mutations should be screened in patients presenting with unexplained reversible leukoencephalopathy, for which SLC13A3 deficiency is a novel differential diagnosis. ANN NEUROL 2019;85:385-395.

α-Ketoglutaramate: an overlooked metabolite of glutamine and a biomarker for hepatic encephalopathy and inborn errors of the urea cycle.

Glutamine metabolism is generally regarded as proceeding via glutaminase-catalyzed hydrolysis to glutamate and ammonia, followed by conversion of glutamate to α-ketoglutarate catalyzed by glutamate dehydrogenase or by a glutamate-linked aminotransferase (transaminase). However, another pathway exists for the conversion of glutamine to α-ketoglutarate that is often overlooked, but is widely distributed in nature. This pathway, referred to as the glutaminase II pathway, consists of a glutamine transaminase coupled to ω-amidase. Transamination of glutamine results in formation of the corresponding α-keto acid, namely, α-ketoglutaramate (KGM). KGM is hydrolyzed by ω-amidase to α-ketoglutarate and ammonia. The net glutaminase II reaction is: L - Glutamine + α - keto acid + H2O → α - ketoglutarate + L - amino acid + ammonia. In this mini-review the biochemical importance of the glutaminase II pathway is summarized, with emphasis on the key component KGM. Forty years ago it was noted that the concentration of KGM is increased in the cerebrospinal fluid (CSF) of patients with hepatic encephalopathy (HE) and that the level of KGM in the CSF correlates well with the degree of encephalopathy. In more recent work, we have shown that KGM is markedly elevated in the urine of patients with inborn errors of the urea cycle. It is suggested that KGM may be a useful biomarker for many hyperammonemic diseases including hepatic encephalopathy, inborn errors of the urea cycle, citrin deficiency and lysinuric protein intolerance.

Alpha-ketoglutaramate: increased concentrations in the cerebrospinal fluid of patients in hepatic coma.

alpha-Ketoglutaramate, a deaminated metabolite of glutamine not previously identified in biological tissues, was measured in the cerebrospinal fluid of human subjects and found to be increased three- to tenfold in patients with hepatic coma. When perfused into the cerebral lateral ventricles of rats, alpha-ketoglutaramate (10 mM) depressed the animals' nocturnal locomotor activity, and at higher doses induced circling behavior and myoclonus. The concentration of alpha-ketoglutaramate in cerebrospinal fluid appears to be a reliable diagnostic indicator of hepatic coma, and its accumulation may contribute to the pathogenesis of this disease.

Severe cognitive impairment correlates with higher cerebrospinal fluid levels of lactate and pyruvate in a canine model of senile dementia.

Diagnosis of dementia of the Alzheimer's type depends on clinical criteria and exclusion of other disorders because, at this time, a validated biological marker, aside from histological brain examination, remains to be established. The canine counterpart of senile dementia of the Alzheimer type (ccSDAT) is considered a promising model for examining behavioral, cellular and molecular processes involved in early phases of human brain aging and Alzheimer disease (AD). In order to investigate the first events taking place in canine cognitive dysfunction, in this paper we established a new and rapid behavioral test that finely discriminates the degrees of cognitive impairment. Cerebrospinal fluid (CSF) analysis was performed to determine the relationship between each disease stage and modification of cerebral energy metabolism. Our results demonstrate a parallel increase of lactate, pyruvate and potassium concentrations in the severe cognitive deficit. These differences are discussed in view of the neuroprotective role presently given to lactate.

Fluorometric determination of pyruvate and alpha-ketoglutarate in cerebrospinal fluid and plasma of infants and children. A simple test that screen for metabolic disorders.

Levels of pyruvate and alpha-ketoglutarate in the cerebrospinal fluid (CSF) of 26 children, aged 4 months to 5 1/2 years, with febrile seizures and of 19 children, aged 4 months to 14 years, with the diagnosis of epilepsy were not different from values seen in 119 "normal" children 8 days to 14 years of age. The CSF samples from 24 adults, 24 to 81 years of age, suspected of having a herniated disk were also examined. In the pediatric age group, the data showed a highly significant downward trend of CSF and plasma alpha-ketoglutarate values with age; pyruvate values did not change. A correlation of the values of the two keto acids in the blood and CSF of 42 other children without apparent neurologic disease was also made. Findings in a child with thiamine deficiency suggest that CSF alpha-ketoglutarate may be a more sensitive indicator of deficiency than plasma alpha-ketoglutarate or pyruvate. Measurements of these keto acids in plasma and CSF may be diagnostically useful in a variety of metabolic disorders. Findings in 155 children from birth (20 minutes) to 17 years of age without neurologic disease are submitted as a standard of reference.

Cerebral metabolism during cross-circulation in experimental hepatic failure in the pig.

The effect of hepatic assistance on cerebral metabolism was evaluated in a series using cross-circulation, anticipating increased efficiency of hepatic support. The experimental model for liver failure was pigs with totally devascularized liver. Cross-circulation with a normal sibling pig, cross-circulation with inflow in the donor directly into the portal vein and cross-perfusion with isolated perfused liver starting 20 h after elimination of liver function in the recipient and lasting for 3 h did not increase survival. Before cross-circulation in these three groups, the cerebral flow and oxygen uptake were decreased; during the cross-circulation a significant but temporary increase was found. In experiments with early and prolonged perfusion with isolated perfused liver no changes in cerebral flow, oxygen or glucose uptake were found, and these variables were still normal 6 h after termination of the perfusion. The survival time was significantly increased. In the control group a significant rise in blood and CSF ammonia was found with a mean CSF/blood ratio of 0.92. After cross-circulation, the CSF/blood ratio was 0.52 and 0.62, respectively, indicating a proportionally greater elimination of ammonia from the cerebrospinal fluid than from the blood. Cross-circulation did not significantly change the alpha-ketoglutarate, glutamate or glutamine CSF concentrations. After prolonged cross-perfusion, the ammonia blood/CSF ratio was 0.24. It is concluded that by extended extracorporeal hepatic assistance it is possible to increase survival and to prevent changes in cerebral metabolism and ammonia accumulating in the cerebrospinal fluid.